Alkene space velocity

III.D.2. Alkanc/ALkene Ratio and Alkene Space Velocity. 274... 

The choice of appropriate reaction conditions is crucial for optimized performance in alkylation. The most important parameters are the reaction temperature, the feed alkane/alkene ratio, the alkene space velocity, the alkene feed composition, and the reactor design. Changing these parameters will induce similar effects for any alkylation catalyst, but the sensitivity to changes varies from catalyst to catalyst. Table II is a summary of the most important parameters employed in industrial operations for different acids. The values given for zeolites represent best estimates of data available from laboratory and pilot-scale experiments. 

Two points are emphasized (i) zeolites can be successfully operated at the same or higher severities (with respect to P/O (feed alkane/alkene) ratio and OS V (alkene space velocity)) than the liquid acids (ii) the productivities of zeolite catalysts (i.e., the total amount of alkylate produced per mass of catalyst) are roughly the same as of that of sulfuric acid. If the intrinsic activities of zeolites (which have 0.5-3 mmol of acid sites per gram) are compared with that of sulfuric acid (which has 20 mmol of acid sites per gram), zeolites outperform sulfuric acid. Nevertheless, the price of a zeolite catalyst and the high costs of... 

The spent acid strength is maintained at about 90 wt% H2SO4. The molar isobutane/alkene feed ratio ranges from 7 1 to 10 1. Typical operating alkene space velocities (LHSV) range from 0.2 to 0.6 h-1 (corresponding to WHSVs from 0.06 to 0.19 h 1). The optimum reaction temperatures range from 279 to 283 K, but some units are operated at temperatures up to 291 K. 

Because of its large reactor volume, the auto-refrigerated process can operate at very low alkene space velocities of about 0.1 h-1 LHSV (WHSV ca. 0.03 h 1). This design helps in increasing the octane number of the product and lowering acid consumption. The reaction temperature is maintained at about 278 K to minimize side reactions. Spent acid is withdrawn as 90-92 wt% acid. The isobutane concentration in the hydrocarbon phase is kept between 50 and 70 vol%. 

The catalyst is reported to be a true solid acid without halogen ion addition. In the patent describing the process (239), a Pt/USY zeolite with an alumina binder is employed. It was claimed that the catalyst is rather insensitive to feed impurities and feedstock composition, so that feed pretreatment can be less stringent than in conventional liquid acid-catalyzed processes. The process is operated at temperatures of 323-363 K, so that the cooling requirements are less than those of lower temperature processes. The molar isobutane/alkene feed ratio is kept between 8 and 10. Alkene space velocities are not reported. Akzo claims that the alkylate quality is identical to or higher than that attained with the liquid acid-catalyzed processes. 

The catalyst is faujasite derived, with a high concentration of sufficiently strong Brpnsted acid sites and a minimized concentration of Lewis acid sites. It also contains a hydrogenation function. The process operates at temperatures of about 323-373 K with a molar isobutane/alkene ratio between 6 and 12 and a higher alkene space velocity than in the liquid acid-catalyzed processes. Preliminary details of the process concept have been described (240). 

Pure component studies indicate the rate of mercaptan formation is sufficiently rapid at hydrotreating conditions compared to the saturation step which lead to alkane [8]. The exothermic reversible reaction, which shifts to the left at higher hydrogen sulfide partial pressure, is also dependent on temperature, feedstock type, total sulfiir, partial pressure of hydrogen and alkenes, space velocity and catalyst type. Furthermore the size of the reactor affect the balance between the kinetic sulfur removal and alkene saturation [9]. 

---